Individuals with multiple sclerosis (MS) often show performance deficits on a range of cognitive tasks, including abstract reasoning, and information processing speed. Furthermore, the cognitive deficits are greater for individuals in more advanced stages of MS. What is not clear from extant research is whether each of the cognitive impairments is a direct, unique consequence of MS. That is, does MS lead directly to diminished attention, memory, reasoning, and processing speed? Or does MS directly affect only one of these processes, which, in turn, leads to impairments in other cognitive skills? For example, perhaps MS affects only memory directly; the other cognitive consequences would be indirect, stemming from the direct effect of MS on memory. The aim of the proposed research is to determine the direct and indirect effects of MS on cognitive processes. 125 30-55 year old MS patients will be tested, along with 125 adults without MS who are matched by sex, age, and education. Each subject will be administered 15 primary instruments: 3 measures of MS symptoms, 3 measures of speed of information processing, 3 measures of short term memory, 3 measures of working memory, and 3 measures of reasoning ability. Structural equation modeling will then be used to compare models that posit different links between MS, processing speed, memory, and reasoning. Specifically, these analyses will be used to distinguish a model in which MS affects speed, memory, and reasoning directly from models in which MS affects some but not all of these cognitive processes. For example, preliminary results suggest that MS-related deficits in memory are due to deficits in processing speed. Consequently, one of the comparison models of interest is one that omits the direct link between MS and memory. These results will help to identify the impact of MS on cognition and thereby help to plan more effective cognitive rehabilitation programs for MS patients. If, for example, memory deficits are actually a byproduct of speed deficits, then rehabilitation should be directed at speed of processing; improved processing speed should also improve both memory and reasoning. The results of this project will help to identify the appropriate targets of rehabilitation efforts.